Characterizing Conformational Distribution in Amorphous Film of Organic Emitter and Its Application in "Self-Doping" Organic Light-Emitting Diode.

Conformational distributions and mutual interconversions of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utlization. However, their influence of photophysics in amorphous film states are still underdevelopment due to the lack of a suitable quantitative analysis method. Herein, by using temperature dependent time-resolved photoluminescence spectroscopy, we quantitatively measured the relative populations of the conformations of a TADF emitter for the first time which not only help deepen the understanding of conformational distributions of TADF emitters, but also provide an aspect to analyze the overall properties of organic amorphous films for other applications. In terms of conformational distribution aspect, we further propose a new concept of "self-doping" for realizing high-efficiency nondoped OLEDs. Interestingly, this "compositionally" pure film actually behaves as a film with dopant (quasi-equatorial form) in a matrix (quasi-axial form). The concentration-induced quenching that may seriously occur at high concentrations is thus expected to be effectively relieved. The "self-doping" OLED by using newly developed TADF emitter TP2P-PXZ as a neat emitting layer, realizes a high maximum external quantum efficiency of 25.4% and neglectable efficiency roll-off. This demonstrates high-performance in doped OLEDs can indeed be achieved with the present "self-doping" emitting layer based on one single compound.